1. Field of the Invention
The present invention pertains to isocyanate compositions containing blowing agents. More particularly, the subject invention pertains to compositions comprising medium viscosity polymeric methylenediphenylenediisocyanates (polymeric MDI) and monochlorodifluoromethane.
2. Description of the Related Art
Polymeric MDI is a valuable commodity whose largest commercial use is in the preparation of rigid polyurethane foams, particularly insulating foams for refrigerators, freezers, and roofing and siding laminate boards. At present, the majority of such rigid foams are blown with monochlorotrifluoromethane, refrigerant 11, a CFC which has come under increasing disfavor due to its ozone depletion potential. Monochlorodifluoromethane, an HCFC, has been proposed as a substitute for R-11, due to the fact that its ozone depletion potential is only c.a. 5% of R-11. However, R-22 is a gas at room temperature while R-11 is a liquid, and thus its use is problematic. Polyol mixtures containing R-22 develop considerable vapor pressure, for example, which can create problems when drums or tank cars are exposed to high temperatures. The atmospheric pressure solubility of HCFC-22 in polyester polyols and polyether polyols, for example, is only about 3 and 5 weight percent respectively.
One possible solution to this problem is to incorporate R-22 blowing agent in both the polyol component and isocyanate component of the polyurethane system. Unfortunately, the vapor pressure in the isocyanate component can still present a problem.